The present invention relates to a process for the oligomerization of alpha olefins in the presence of a boron trifluoride/alcohol catalyst with a carboxylic acid used as a modifier to increase the amount of trimer and tetramer in the product.
The oligomerization of alpha olefins in the presence of a Lewis acid catalyst has been used commercially to produce synthetic fluids which are useful for various commercial applications, such as in synthetic lubrication oils, drilling fluids, hydraulic fluids, and heat transfer fluids. The catalyst of choice is usually boron trifluoride combined with a protic promoter, such as butanol or water. See, for example U.S. Pat. Nos. 4,956,512 and 5,945,574. U.S. Pat. No. 3,769,363 describes the use of an oligomerization catalyst containing boron trifluoride with a carboxylic acid having more than three carbon atoms in the molecule as the promoter. In order to control the degree of oligomerization and increase the amount of trimer in the product, U.S. Pat. No. 3,997,621 teaches the use of an ester in the reaction mixture as a modifier. By controlling the degree of oligomerization the resulting product will have a narrower range of carbon atoms in the molecule which results in more controlled volatility and temperature viscometrics. Generally speaking, it is desirable to produce a product having low volatility and low viscosity for use in crankcase oils.
Surprisingly, it has been found that carboxylic acids may be used as a modifier in the oligomerization reaction of alpha olefins to increase the yields of trimer and tetramer in the product when used in combination with a boron trifluoride/alcohol catalyst complex.
The present invention is directed to an improved process for controlling the oligomerization of an alpha olefin in the presence of a catalyst complex comprising boron trifluoride and an alcohol to form a polyalphaolefin product, the improvement comprising including with the catalyst complex in the reaction zone containing the catalyst complex and the alpha olefin a sufficient amount of a carboxylic acid modifier to significantly increase -the amount of trimer and tetramer present in the polyalphaolefin product formed as compared to an oligomerization process in which the carboxylic acid is not present, wherein the carboxylic acid contains from 2 to about 10 carbon atoms. The alpha olefin monomer may contain anywhere from 3 to about 22 carbon atoms in the molecule but will preferably contain from 3 to about 14 carbon atoms in the molecule. The oligomerization reaction will proceed over a broad temperature range but is usually carried out in the temperature range of from about 30xc2x0 C. to about 150xc2x0 C. and most preferably within the temperature range of from about 40xc2x0 C. to about 60xc2x0 C. Following oligomerization the product may be left unsaturated or it may be hydrogenated to saturate the double bonds remaining in the molecule depending on the application for which the product is to be used.
The carboxylic acid used as a modifier according to the present invention should contain from 2 to about 10 carbon atoms in the molecule. The carboxylic acid may contain unsaturated carbon to carbon bonds and the carbon chain may be either branched or unbranched. Examples of suitable modifiers include, but are not necessarily limited to, acetic acid, propionic acid, butyric acid, valeric acid, caproic acid, enanthic acid, caprylic acid, pelargonic acid, and capric acid. Particularly preferred for carrying out the invention is acetic acid.
The amount of carboxylic acid present in the oligomerization mixture should be at least a sufficient amount to significantly increase the yield of trimer and tetramer in the final product as compared to an oligomerization mixture which does not contain the carboxylic acid. In general, a significant increase should be at least a 20 weight percent increase in the amount of trimer and tetramer in the product as compared to the oligomerization reaction without the modifier present. The precise amount of carboxylic acid modifier required to practice the invention will vary somewhat depending upon the carboxylic acid chosen, the alpha olefin, the temperature of the reaction mixture, the amount of catalyst, and the ratio of the boron trifluoride to the alcohol promoter. However, these variables merely require the optimization of the conditions of the oligomerization reaction and, as such, should not require any more than routine experimentation which is well within the ability of one skilled in the art.
In general, the amount of carboxylic acid modifier will usually be present within the range of from about 0.08 mole % to about 2.0 mole % of modifier to olefin with about from 0.16 mole % to about 0.35 mole % of modifier being preferred.
The alpha olefin used as the feed for the oligomerization reaction will usually, contain from 3 to about 22 carbon atoms in the molecule and preferably will contain from 3 to about 14 carbon atoms in the molecule. The alpha olefin, of course, will contain an unsaturated carbon to carbon bond in the 1-position, however, the molecule may also contain other internal unsaturated carbon to carbon bonds in the molecule. In addition, the alpha olefin may be either branched or unbranched. However, in most instances when practicing the invention the alpha olefins will be mono-olefinically unsaturated, that is, they will contain only a single double bond in the 1-position, and will be unbranched, that is, the feed will be comprised of a normal alpha olefin. The present invention is most advantageous when the alpha olefin feed is a relatively pure alpha olefin, that is, the feed consists primarily of one alpha olefin as opposed to a mixture of different alpha olefins. Examples of alpha olefins which are suitable for use as feeds in practicing the present invention include propene, 1-butene, 1-pentene, 1-hexene, 1-heptene, 1-octene, 1-decene, 1-dodecene, or 1-teradecene.
The alcohol promoter will be an alcohol having from 1 to about 24 carbon atoms in the molecule, more typically having 12 carbon atoms or less in the molecule. Operable protic promoters include, but are not limited to, methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-methyl-1-propanol, 1-hexanol, and the like. Particularly preferred as a protic promoters are 1-propanol and 1-butanol.
The amount of boron trifluoride used in the reaction mixture is usually in molar excess relative to the amount of the alcohol promoter. The reaction vessel is typically pressurized with boron trifluoride or a mixture of boron trifluoride and nitrogen. The boron trifluoride may be bubbled through the reaction mixture, introduced as part of the catalyst complex, or otherwise introduced into the reaction mixture, as for example, by mechanical stirring.
Although the oligomerization reaction will proceed over a wide temperature range, the reaction preferably is carried out at a temperature in excess of 30xc2x0 C., and preferably is carried out at a temperature in excess of 40xc2x0 C. The upper temperature limit is about 150xc2x0 C., but as a practical matter the oligomerization reaction mixture is usually maintained at a temperature of less than about 60xc2x0 C.